The accurate placement of catheters and other medical devices has become important in recent years for the treatment of a variety of illnesses, for example, the balloon angioplasty of coronary and peripheral arteries. There are many new uses that will increase the need for precise catheter placement which include stricture dilation performed in the prostrate, selective infusion of drugs to specific sites, and a host of other applications. Catheter placement has been done almost exclusively under x-ray guidance. In such techniques the contrast medium is sent through the catheter while it is being observed by x-ray fluoroscopy, the contrast moving through the catheter and the jet at the tip showing the location. The physician frequently a "interventional radiologist" guides the catheter between x-ray sightings. Limitations on catheter placement by x-rays include the safety issues based on x-ray dosage which were received by the patient and physician and the contrast medium load on the patient's kidneys. Other limitations are related to the need for the procedure to take place in a special room and the need for a radiologist to also be in attendance during the procedure. As one can ascertain both of these requirements add inconvenience and expense to the procedure. Ultrasonic imaging can provide excellent images of many of the blood vessels and prostatic urethra in which these procedures take place. In addition to visualizing the vessels, the Doppler capability of ultrasonic imaging allows measurements of flows achieved by the catheter, as to whether to augment flow or diminish it. There has not been any reliable method of localizing a catheter's position by ultrasonic imaging. This is because the ultrasonic image of the catheter depends on the angle of the catheter to the ultrasonic beam as the catheter moves in the vessel. Some portion of it may become visible, but which portion and when it becomes visible depends on the exact path of the vessel imaged and the particular location of a isonifying ultrasonic source. For these reasons no one uses ultrasound to try to accurately place a balloon catheter, for example, within a peripheral artery even though the artery and the blockage are visible by ultrasonic imaging. The prior art was cognizant of the use of ultrasound imaging to locate the tip of a needle.
Reference is made to U.S. Pat. No. 4,249,539 issued on Feb. 10, 1981 to David H. R. Vilkomerson et al and entitled ULTRASOUND NEEDLE TIP LOCALIZATION SYSTEM. This patent describes a means for localizing a needle tip using an ultrasound imaging system. The apparatus consists of a small transducer whose location was to be determined, receiving the transmitted pulse of an ultrasonic imaging system and sending it back to the imaging system to show its location. In particular, the patent described the method whereby the return signal was generated electronically and returned directly to the imaging system rather than acoustically back through the body. More general uses of this technique, for example, locating interventional devices in an artery by means of catheter carried transducers are being contemplated. Such uses require an accurate method of showing the position of the localizing transducer. The technique described had various associated problems. In the method described in the patent, a comparator was employed to measure the received signal compared to a reference level. When the received signal was higher the apparatus would generate a pulse that would, after being delayed a time equal to that used in propagating from the ultrasound system to the transducer (i.e., simulating the time required of a pulse reflected by the localizing transducer if direct reflection were used), be inserted into the signal stream of the ultrasonic imaging system, causing a "dot" to appear at the location of the transducer. A problem with that method is that the reference level must be constantly re-adjusted for varying signal levels. When the transmitting transducer is close to the localizing transducer, not only the main lobe but the side lobes of the imaging beam will exceed the reference level, causing a "dot" to appear in several beam positions of the image resulting in a smear in the image.
If the reference level is reduced to obtain the single beam position that represents the center of the imaging beam and which produces the most accurate location of the localizing transducer, a slight change in the position may reduce the signal level at the localizing transducer enough to eliminate any signal whatsoever, thus erasing the "dot". Thus the prior art required a need for continuous readjustment of the reference level to achieve localization and is a serious drawback to system operation. Also in regard to this particular technique, see an article entitled ULTRASONICALLY MARKED CATHETER--A METHOD FOR POSITIVE ECHOGRAPHIC CATHETER POSITION IDENTIFICATION by B. Breyer, et al and published in The Medical and Biological Engineering and Computing Journal, May 1984, Pages 268-271. As one will understand from that article, the system described has the above-noted disadvantages.
It is therefore an object of the present invention to provide apparatus to be employed with an ultrasound system which eliminates the need for readjustment of any kind while achieving the optimum localization of the receiving transducer position.
A further object of the present invention is to provide an accurate and improved system for determining the position of an ultrasonic transducer.